10 Circuit Components You Should Know

Chefs have their miscellaneous ingredients, like condensed milk, cream of tartar, and xanthan gum. As engineers, we too have quite our pick of circuits, and a good circuit designer should know what's out there. Not just the bread and butter ingredients like resistors, capacitors, op-amps, and comparators, but the miscellaneous "gadget" components as well.

Here are ten circuit components you may not have heard of, but which are occasionally quite useful.

7400 series logic has undergone a metamorphosis over the past few decades, from fundamental logic building blocks in 14-20pin DIP packages, to single-gate glue logic in small low-count SOT-23-6 and SC-70-6 packages. TI introduced two parts which are "multifunction gates" -- they have an odd truth table based on 3 inputs that allows them to be used as AND gates, OR gates, inverters, buffers, multiplexers, etc. depending on how the pins are connected. They are now second-sourced by Fairchild, NXP, and ON Semiconductor.

Why use these parts?

The last circuit design I worked on needed several glue logic parts. We could have used 1G04 inverters, 1G32 OR gates, 1G00 NAND gates, and 1G00 AND gates. Instead, we used 1G97s and 1G98s. We needed to store fewer different parts because of this.

If you have a small stash of these two gates available, they're there in a pinch when you need them.

If I had a production design with only one NAND gate and no other logic gates, I'd use a 1G00: it's cheaper than the 1G97. But otherwise, they're like a Swiss Army knife for glue logic.

2. TL431 adjustable shunt regulator

If you look at the schematic of any isolated-output power supply, chances are it uses a TL431 and an optoisolator. The TL431 essentially acts like an NPN transistor or N-channel MOSFET but with a precision 2.5V VBE/VGS drop. In the TL431, the pins are called cathode (K), ref (R), and anode (A). The K and A pins are analogous to drain and source of a MOSFET, and R is analogous to the gate

If that doesn't mean anything to you, think of it as a precision comparator:

When the reference pin is more than 2.5V above the anode, the part begins to conduct between cathode and anode.

You can use this as a precision comparator, or as a shunt reference (by connecting a resistor divider across cathode/ref/anode pins). In switching power supplies, they're usually used as a feedback regulator: a voltage divider connects to the ref and anode pins, with an optoisolator diode connected in series with the cathode, so that when the power supply output reaches its regulation point, the TL431 begins to conduct through the optoisolator, causing the power supply controller to reduce its output. You'll usually see a capacitor connected between ref and cathode pins to stabilize the feedback loop.

Many manufacturers have TL431 equivalents, and the parts are inexpensive.

There are also several "low-voltage" variants like the TLV431, TS431, and LMV431 which have thresholds of 1.24V rather than 2.5V. These are also useful, but beware of the limitations on cathode-to-anode voltage: the TLV431 and TS431 only work up to 6V; if you want to go higher you need to use the TLVH431 which goes up to 18V. The LMV431 works up to 30V cathode-to-anode voltage. All of these components have lower (better) specifications on reference leakage current than the plain TL431 (0.5uA max instead of 4uA; the TS431 has specs of 0.16uA!), and lower minimum cathode-to-anode operating current (60-100uA max instead of 1mA). If I were using a micropower circuit, I'd turn to these parts first because of their better performance, although the TL431 is less expensive.

If you have to design a 1Hz oscillator without using a microcontroller, your first instinct might be to use a 555 oscillator. It will work, but not well -- if you try, you'll find that you either have to use a very large resistor or a very large capacitor. If you use a large resistor (>1 megaohm), the oscillator frequency is prone to variation from leakage current from the 555 or from surface contamination on the circuit board. If you use a large capacitor (>1uF), it may be physically large, or expensive, or suffer from poor tolerance or high internal leakage.

A better approach is to use a chip like the HC4060 and a 32.768kHz watch crystal, and not have any of the tolerance issues of an RC oscillator -- 32kHz quartz crystals are usually specified at 20ppm accuracy. The 4060 contains an oscillator driver (just an inverter and buffer) and a 14-stage counter. With the oscillator driver, you can drive an RC circuit or a crystal. The counter divides down the input to lower-frequency outputs, and gives you 10 taps out of the 14 stages -- with 32.768kHz source frequency, the 4060 yields all power-of-2 frequencies from 2.048kHz down to 2Hz with the exception of 16Hz. (You could get 1Hz by taking the 2Hz output and putting it through a flip-flop.)

4. Ceramic resonators

Sometimes you need a frequency source that is more accurate than an RC filter, but does not need to be as accurate as a quartz crystal.

Ceramic resonators are two-terminal or three-terminal resonators that are about 0.5% accuracy; better than the 5% best-case accuracy you get from RC filters (capacitors with better than 5% tolerance are expensive), but poorer than the 50ppm or better you can get from a quartz crystal. The three-terminal variety have built-in capacitors for ensuring proper oscillation. The typical frequency range is 200kHz - 50MHz.

From what I remember, ceramic resonators used to be less expensive than crystals. I just checked on Digikey, and this doesn't seem to be the case anymore -- they're about the same; slightly more expensive for surface mount but slightly less expensive for through-hole. Ceramic resonators do have one major advantage over crystals: they're physically smaller, with 4mm x 2mm size as an ordinary option for SMT ceramic resonators (including built-in capacitors!), whereas the smallest normally available crystal size is about 8.5mm x 3mm.

Ceramic resonators are not appropriate for timekeeping -- a 0.5% accurate frequency source could gain or lose 7 minutes per day! -- but one application which can use them is a UART. If you have a microcontroller application, where the only frequency accuracy requirement is communicating over RS232, 0.5% accuracy is plenty. UART bytes with start and stop bits have 10 bits per byte, so 5% error in the frequency translates to half a bit per byte, and 0.5% error translates to 1/20 of a bit per byte. RC accuracy is marginal for this sort of application, but ceramic resonators are fine.

5. Switched-capacitor charge-pump inverter (ICL7660 or equivalent)

Let's say you have a 5V, 1A power supply and you'd really like a -5V supply for one or two analog circuits. Intersil invented a part over 20 years ago that meets the bill: the ICL7660.

This part takes an external capacitor and alternates switching it between two connection paths. If you make one of these paths the 5V power supply and the other the -5V supply rails, then the capacitor charges up across the 5V supply and discharges across the -5V load.

For a few milliamps, it works fine. Many manufacturers make a 7660 circuit, so there are multiple sources.

But it's not a regulated power supply. The output impedance of the 7660 is a function of the onboard switch resistance, the external capacitor value, and the switching frequency. The more current you draw from the 7660, the more the voltage will droop towards zero.

It also is a switching converter, both drawing and delivering pulses of charge when the capacitor switches, so it produces noise at the switching frequency.

Still, it's a useful part to have in your back pocket for that one circuit on your board that needs a negative power supply.

There are many schemes for sending serial data between separate devices. Most of them use differential signaling. The only major one that does not is RS-232, which has practical limitations on baud rate if there is noise present. The others include RS-422, RS-485, CAN, and LVDS. These all use differential pairs, so all are far less sensitive to common-mode noise that is induced on both signals. LVDS is frequently used to send digital data between computers and monitors. It uses a lower-voltage signal swing than the levels used in CAN or RS-485, so the power usage and the emitted noise are much lower.

If you have signals you need to transmit from one circuit board to another, that need good noise rejection, you may wish to use LVDS to encode/decode them on each end. Some examples are SPI or PWM signals -- both are sensitive to glitches.

7. I/O expanders

So you've got a microcontroller in an application that's still adding new features, and as you add more and more inputs or outputs, you're running out of pins.

There are chips available that can be controlled over I2C or SPI that will handle less-frequently-changing inputs and outputs. My favorite is the MCP23017 and MCP23S17. These are 16-pin devices; the 23017 is I2C and the 23S17 is SPI.

If you need something for outputs only that's less expensive and less complicated, you can get away with using 74HC595 shift registers on a SPI bus. Each 595 is an 8-bit output that has its inputs shifted in on the SPI clock signal and latched into its outputs on the rising edge of the CS signal.

8. High-side current sense amplifiers

There are times when you have to measure the current into a load circuit and you can't interrupt the ground connection. This leaves you with high-side current sensing. There are a few options for high-side current sensing:

integrated circuits with built-in sensing elements

hall-sensor based current sensors

current sense resistors + amplifiers

The last of these is probably the least expensive and most flexible. I like the ZXCT series of current sensors introduced by Zetex, which was acquired by Diodes Inc. These are used to detect unidirectional currents and produce a current output proportional to the sensed voltage. Their bandwidth is pretty decent and the quiescent current is in the 2-5uA range. The idea is that you use a series resistor to turn the output current into a voltage, and then do with it what you wish (e.g. buffer it). I tried investigating a number of alternative approaches from other vendors in 2008 or 2009, and ended up concluding this was the best performing option for a reasonable price.

They are not useful for bidirectional currents (e.g. charging/discharging batteries) unless you use two devices, one in each direction; this is a much more difficult problem to solve with one device.

9. Current sense resistors

There is an art to choosing current sense resistors. You want a resistor that produces as small a voltage as possible so it doesn't interfere with the rest of your circuit... but on the other hand, you want one that produces as large a voltage as possible so your signal is large enough to use. It may need to dissipate several watts if the current it's measuring is large, but you want it to be inexpensive and small. Oh, and it needs to be accurate.

If your currents are below 100mA, it's not that hard of a problem to solve (1%, 1 ohm or 0.5 ohm resistors capable of dissipating more than 10mW are not very hard to find).

Sometimes you need to switch analog signals, whether it's to expand the number of inputs into an analog-to-digital converter, or choose different gain resistors, or something else.

I divide the kinds of multiplexers into four groups:

A. CD405x / 74HC405x multiplexers and 4066 switches

The standard digital logic series (CD4000 = 5-15V, 74HC = +/-2-6V) contain analog switches. The 4051 is an 8:1 analog multiplexer; the 4052 is a dual 4:1 analog multiplexer; and the 4053 is a triple 2:1 analog multiplexer; the 4066 is a quad switch. These are inexpensive and made by several manufacturers. But they have drawbacks; the leakage current is spec'd at 1uA and the on-resistance usually runs in the tens or low hundreds of ohms.

B. "Improved" 405x with better leakage/on-resistance specifications

You'll need these if you have high-impedance inputs or on-resistance is an important requirement.

Maxim was the first company to introduce a pin-compatible series of switches/multiplexers (MAX4051/2/3) with improved specifications (leakage current less than 0.1uA, on-resistance of